Footnote Sync Architecture

Goal

Define the most performant architecture for synchronizing footnote references, definitions, and hover preview content in Plate.

Reference Scan

ProseMirror

• Official example: Editing footnotes
• Their model is not markdown-style reference plus separate definition block.
• Instead, footnote is an inline atom with its own inner sub-editor.
• The important architectural lesson is not the UI shape. It is this:
  • keep one canonical content source
  • do not duplicate that content into every visible marker
  • propagate changes between outer and inner editors by transaction mapping, not by string mirroring

Tiptap

• Tiptap local repo scan did not surface an official footnote extension.
• Official docs search also did not turn up a built-in footnote feature page.
• Practical reading: Tiptap does not give us a stronger ready-made footnote sync design than ProseMirror itself.

Milkdown

• Milkdown ships footnote_reference and footnote_definition schema support in preset-gfm.
• It does not appear to ship a richer sync / preview / navigation layer on top.
• Practical reading: Milkdown confirms the basic node split, but not the performance architecture above it.

Decision

Plate should use a normalized footnote registry model.

Canonical Data

• Definition content is the single source of truth.
• References must never store mirrored preview text, markdown prefix text, or copied definition content.
• Hover preview is derived from the definition node at render time.

Indexing

• Add one footnote registry per editor instance.
• Registry shape:
  • definitionByIdentifier: Map<string, PathRef>
  • referencesByIdentifier: Map<string, PathRef[]>
  • optional later: referenceByInternalId: Map<string, PathRef>
• The registry should be invalidated on structural or identifier-changing operations that touch footnote nodes.
• On next read, rebuild once and reuse.

Why This Wins

• O(1) lookup from reference to definition after index build.
• No N references each rescanning the whole document.
• No denormalized copies to keep in sync.
• No eager recomputation for every keystroke when footnotes are untouched.
• Keeps hover preview, backlink navigation, and insert flows on one ownership seam.

Recommended API Shape

• editor.api.footnote.definition({ identifier })
• editor.api.footnote.references({ identifier })
• editor.api.footnote.definitionText({ identifier })
• later:
  • editor.api.footnote.registry()
  • editor.api.footnote.hasDefinition({ identifier })
  • editor.api.footnote.isResolved({ identifier })

Implementation note:

• definitionText should read through the registry, not rescan the whole document.

Normalization Rules

• At most one definition per identifier.
• References may share an identifier.
• Missing definitions are allowed transiently during editing, but should be easy to detect and expose through API.
• Definition body stays pure content. Markdown prefixes are serializer-owned.

Navigation Rules

• Reference mod-click:
  • look up definition through registry
  • select the start of the definition body
  • scroll into view
• Backlink click:
  • look up references through registry
  • default to first reference in document order for now
  • select the reference atom itself
  • scroll into view

Hover Preview Rules

• Reference hover should render from canonical definition content.
• Hover UI stays in app/plugin render layer.
• Data lookup stays in package API layer.
• If definition is missing, show no preview or an unresolved state, but do not invent content.

Explicit Non-Recommendations

• Do not copy definition text onto references.
• Do not store hover-preview text in plugin options or React state as a second source of truth.
• Do not recompute definitionText by full-document scan in every reference component render.
• Do not move to ProseMirror's inline-subeditor footnote model. It does not fit Plate's current markdown contract.

Best Next Implementation

1. Add the registry/index layer in @platejs/footnote.
2. Make definition, references, and definitionText resolve through it.
3. Add unresolved / duplicate-definition helpers.
4. Keep current insert and navigation helpers on top of the registry.
5. Only after that, harden hover preview and backlink UX.